Effect of Mobile Phones on Life

Little's Theorem
Little's Theorem (sometimes called Little's Law) is a statement of what was a "folk theorem" in operations research for many years:

N = λTwhere N is the random variable for the number of jobs or customers in a system, λ is the arrival rate at which jobs arrive, and T is the random variable for the time a job spends in the system (all of this assuming steady-state). What is remarkable about Little's Theorem is that it applies to any system, regardless of the arrival time process or what the "system" looks like inside. Proof: Define the following:

α ( t ) ≡ number of arrivals in the interval (0,t ) δ ( t ) ≡ number of departures in the interval (0,t ) N ( t ) ≡ number of jobs in the system at time t = α (t ) − δ( t )